The invention relates to a method for separating ruthenium from noble metal solutions.
In U.S. Pat. No. 4,390,366 a method is disclosed for the separation of ruthenium from noble metal solutions by warming the noble metal solution containing ruthenium and H+ and Clxe2x88x92 ions and then adding chlorate, and further heating the resultant suspension/solution to 80 to 90xc2x0 C. and capturing the ruthenium tetroxide that has formed.
The separation of the noble metal ruthenium from noble metal solutions can be performed by the oxidation of alkali ruthenate in aqueous solutions (see DE 39 35 798 A1), in which the oxidation is performed with ozone at pH levels above 8. alternatively, the ruthenium-containing noble metal solution can be distilled by means of sodium chlorate and/or chlorine gas.
A disadvantage in this process is the fact that, for a complete separation of ruthenium, a very great expenditure of time and energy is necessary, according to the composition of the noble metal solution.
On this account the problem arises of at least partially overcoming the above-stated disadvantages by means of a novel method. The resultant problem is especially to devise a method in which a complete separation of ruthenium from a noble metal solution can be accomplished in a very short time and therefore in a cost-effective manner.
In the method of the invention, in a first process step a noble metal solution containing ruthenium is heated to a temperature of T=50xc2x0 C. to T=65xc2x0 C., while its H+ concentration and Clxe2x88x92 concentration is greater than 1 mole per liter each.
The chloride ions serve to destroy the chlorine dioxide which can form under certain circumstances and has a decidedly strong tendency to self-detonate.
Then, in a second process stage, chlorate is added to the solution conditioned in step a), and the resultant suspension/solution again has a temperature of T=50xc2x0 C. to T=65xc2x0 C. As a rule, during the addition or else also after the addition of chlorate, a carrier gasxe2x80x94air, for examplexe2x80x94is passed through the suspension/solution and the ruthenium tetroxide that has formedxe2x80x94in an absorption solution for example, especially a hydrochloric acid solutionxe2x80x94is captured, and after the addition of the chlorate the suspension/solution is heated to a temperature of T=80xc2x0 C. to T=90xc2x0 C.
The carrier gas serves to greatly dilute the ruthenium tetroxide which, in greater concentration, tends to detonate.
Lastly, in a third process step, bromate is added to the suspension/solution at a pH of 1 to 3, the resultant suspension/solution having a temperature of T=60xc2x0 C. to T=95xc2x0 C. Simultaneously, or thereafter, a carrier gas is passed through the suspension/solution and the ruthenium tetroxide that forms is captured, for example, in an absorption solution, especially a HCl solution.
The pH range of 1 to 3 is important for this last process step, since in this range the ruthenium can very easily be oxidized to ruthenium tetroxide.
First it is advantageous, as it has been proven, if the ruthenium tetroxide is captured in an absorption solution, for example in an HCl solution.
If, when the chlorate is added, an equimolecular chlorate is added to the solution, this is advantageous, since at least in this way the minimum amount is provided for a virtually complete reaction.
It is furthermore advantageous that hydrochloric acid is used as an absorption solution, since solid ruthenium chloride and ruthenium metal can be produced relatively easily from this solution. Advantageously, chlorate in the form of a sodium chlorate solution has proven good for addition to the corresponding solution.
Especially it is advantageous if a 4.5 molar sodium chlorate solution is added, since when chlorate is added as oxidant, due to this highly concentrated solution, the increase in the volume of the reaction solution is relatively slight.
Furthermore it is advantageous to add the bromate in the form of a sodium bromate solution to the solution, especially if this sodium bromate solution is 1.5 to 2 molar, since the increase in the volume of the reaction solution remains slight.
To a special degree it has proven especially advantageous in practice if before the addition of bromate (i.e., in the first process step) is heated to a temperature of T=65xc2x0 C., since the oxidation of the ruthenium to ruthenium tetroxide takes place relatively rapidly, but the self-degradation of the oxidant takes place still relatively slowly.
It is furthermore advantageous if the solution is free of organic compounds before the addition of chlorate (i.e., in the first process step) since thus no uncontrolled oxidation reactions can occur and lead, under certain circumstances, to an explosion.
In this connection it is furthermore advantageous if the rate of addition of the chlorate solution is adapted to the progress of the reaction, so as to prevent excesses of chlorate and thus to prevent a violent reaction.
It is advantageous if, after the addition of bromate and the reaction are finished an H+ concentration of c greater than 1 mol/l is established in order thus to dissolve any metal hydroxides that have formed in any way, which otherwise would interfere with the further processing of the now ruthenium-free solution.
In an advantageous manner, after the adjustment chlorine gas is introduced into the solution in order to remove bromide and bromine from the now ruthenium-free solution. If desired in this case it can be strongly acidified again with HCl.
Lastly it is advantageous to use compressed air simultaneously with the introduction of chlorine, since this provides a mechanical support and thus a more intimate gassing of the solution with chlorine, and the bromine that simultaneously forms is driven out of the solution.